The present invention relates to a measuring device, and more particularly to a protractor used for measuring the angle of a slant surface of an object.
None of the prior art protractors is equipped to measure an angle formed by the surface of an object and a horizontal surface against which the object is set. As shown in FIG. 1, a level 10 of the prior art is composed of a main body 12 having a plane surface 14 and three air tubes 16. One of the three air tubes 16 is perpendicular to the plane surface 14, while the second air tube 16 is parallel to the plane surface 14. The third air tube 16 is arranged in such a manner that it forms an angle of 45 degrees with the plane surface 14. In using the level 10, the main body 12 of the level 10 is placed on the surface of an object intended to be measured. The angle formed by the measured surface of the object, which is set against a horizontal surface, is determined by the user of the level 10 by observing that the position of an air bubble 17 is at the center of the air tube 16. For example, if the air bubble 17 is located at the center of the air tube 16 perpendicular to the plane surface 14, the measured surface of the object is perpendicular to the horizontal surface against which the object is set. In other words, a right angle is formed by the measured surface and the horizontal surface in this instance.
The prior art level 10 described above is limited in that it measures only those angles which are consistent with the predetermined angles formed by the plane surface 14 and the air tubes 16. It is thus apparent that the level 10 can not be used to take the measurement of an angle that is different from any one of those predetermined angles formed by the plane surface 14 and the air tubes 16 of the level 10.